Recent concerted efforts in the field of liquid crystal materials have yielded a new class of reflective, cholesteric texture materials and devices. These liquid crystal materials have a periodic modulated optical structure that reflects light. The liquid crystal material comprises a nematic liquid crystal having positive dielectric anisotropy and chiral dopants. These materials, known as polymer stabilized cholesteric texture (PSCT) and polymer free cholesteric texture (PFCT) are fully described in, for example, U.S. Pat. No. 5,251,048 and patent application Ser. Nos. 07/694,840 and 07/969,093, the disclosures of which are incorporated herein by reference.
Reflective cholesteric texture liquid crystal displays (both PSCT and PFCT) have two stable states at a zero applied field. One such state is the planar texture state which reflects light at a preselected wavelength determined by the pitch of the cholesteric liquid crystal material itself. The other state is the focal conic texture state which is substantially optically transparent. By stable, it is meant that once set to one state or the other, the material will remain in that state, without the further application of an electric field. Conversely, other types of conventional displays, each liquid crystal picture element must be addressed many times each second in order to maintain the information stored thereon. Accordingly, PSCT and PFCT materials are highly desirable for low energy consumption applications, since once set they remain so set.
The configuration of LCDs using PSCT and PFCT materials is substantially the same as in conventional passive LCDs: picture elements (pixels) are addressed by crossing lines of transparent conducting lines known as rows and columns. Conventional methods for addressing or driving such displays can be understood from a perusal of FIGS. 1 and 2. FIG. 1 illustrates a table showing the state of the liquid crystal material after the application of various driving voltages thereto. The liquid crystal material begins in a first state, either the reflecting state or the non-reflecting state, and is driven with an AC voltage, having an rms amplitude above V.sub.4 in FIG. 1. When the voltage is removed quickly, the liquid crystal material switches to the reflecting state and will remain reflecting. If driven with an AC voltage between V.sub.2 and V.sub.3 the material will switch into the non-reflecting state and remains so until the application of a second driving voltage. If no voltage is applied, or the voltage is well below V.sub.1, then the material will not change state, regardless of the initial state. It is important to note however, that the application of voltages below V.sub.1 will create optical artifacts (as discussed in greater detail hereinbelow), but will not cause a switch in the state of the material.
The conventional method of driving PSCT and PFCT displays is described in an article entitled "Front-Lit Flat Panel Display from polymer Stabilized Cholesteric Textures", by Doane, et al. and published in Conference Record, page 73, Japan Display '92, Society of Information Displays, October 1992 (the "Doane Article"). The Doane Article teaches addressing a row in a display by applying an AC waveform with an rms amplitude V.sub.rs between V.sub.2 and V.sub.3. A column voltage of zero is applied to the columns of all the pixels in the rows which are to be in the non-reflecting state. An AC voltage with rms amplitude greater than or equal to V.sub.4 -V.sub.rs, but less than V.sub.1 is applied to the columns of all pixels which are to be in the reflecting state.
The column voltages are out of phase with respect to the row voltages so that the effective voltage across the selected pixels is greater than or equal to V.sub.4. The amplitude of the column voltage is always less than V.sub.1, thus as the addressing of the display progresses from row-to-row, the column voltage does not alter the state of the pixels in rows which have already been addressed. This may be appreciated from a review of FIG. 2. Specifically, for a given single pixel, at time t.sub.1 no voltage is applied to the row address line of the display for the pixel, and a column voltage of V.sub.c (either + or -). The result is no change in the pixel since the pixel's row was not selected. During time t.sub.2 no voltage is applied to either the row or column lines for the pixel, and again the pixel is unchanged.
During time t.sub.3 however, a voltage of V.sub.rs (either + or -) is applied to the pixel row address line, and a voltage of V.sub.c (either + or -) is applied to the column address line. As a result, the pixel is driven to the reflecting state as shown in FIG. 1. During time t.sub.4, a voltage of V.sub.rs (either + or -) is applied to the pixel row address line, and no voltage is applied to the column address line. As a result, the pixels is driven to the non-reflecting state.
While this method of driving PSCT and PFCT displays has been the accepted standard, it nonetheless possesses several characteristics which have rendered it increasing untenable for commercial applications. For example, while the image on the display is being updated, the display shows annoying optical artifacts from the previously displayed information. The electro-optical curve of the reflecting state measured with voltage on is different than with voltage off. Moreover neither curve is ideally fiat between zero volts and V.sub.1. Thus, as columns are being addressed, the reflectance of the material will vary slightly, resulting in an undesirable flickering of the display. This flicker increases as the voltage applied along the columns is increased, thus driving pixels, even in unselected rows, closer to V.sub.1.
Moreover, in this type of LCD, the following mathematical relationship must be maintained in order to achieve consistent uniform addressing: EQU V.sub.1 &gt;V.sub.4 -V.sub.3
As described herein, V.sub.4 is typically about 40 volts, V.sub.3 is typically about 34 volts, and V.sub.1 is typically about 10 volts. However, cell spacing, actual material composition, and temperature all substantially impact actual voltage requirements. Thus, a large scale, commercially producible display is not readily producible. This is because there is not a sufficient voltage margin as required for production tolerances. Further, for displays that operate in particular areas of the spectrum, (for example yellow) the prior art driving scheme will no work since they exhibit large hysteresis, hence larger (V.sub.4 -V.sub.3) or a lower V.sub.1.
Moreover, the driving scheme of the prior art has not been adapted to completely eliminate residual memory effects from images that have been retained on the display for some time. Specifically, prior art attempts to deal with residual image memory effect required combining several cycles of AC voltage to write a new row of information, writing the information to the entire display concurrently, and increasing the cycle time of the AC voltages applied. These attempts however, did not resolve the problems of residual memory effects. Moreover, they are distracting to the viewer, as the cycle time for this process is approximately 100 milliseconds.
Thus, there exists a need for an improved scheme for driving or electronically addressing a PSCT or PFCT LCD. Such a scheme should be easily integrated into such devices, and provide for effective addressing of large, color displays.